The demands on modern drive systems are growing more and more. In a pick-and-place machine, for example, smaller and smaller components must be placed more and more precisely on a printed circuit board. In this context, the number of components per unit area of printed circuit board is increased continually by miniaturization such that the speed of the component positioning must continue to increase as well in order to maintain as high a throughput as possible in such a machine. Higher positioning accuracy at simultaneously shorter positioning times are contrary objectives, which can only be achieved by optimally parameterized control loops in conjunction with high-quality motors and position-measuring systems.
However, in order to be able to set the parameters of a control loop in an optimum manner, it is important that the knowledge of the drive system is as accurate as possible. One tool for analyzing a drive system is the ascertainment of the transfer function of the drive system or of a transformer within the drive system. This transfer function describes the attenuation and the phase shift which a signal of a particular frequency applied to the input of the transformer experiences up to the output. The ascertainment of the transfer function of the drive system or of the transformer (which ascertainment is also referred to as identification) should be performed for the open control loop, since knowledge of this “open loop” transfer function of the target system allows one to make an assertion regarding the stability of the drive system. As described below, the “open-loop” transfer function can be ascertained even in the case of a closed control loop.
To identify a control loop or more generally a transformer, an input signal is applied to the input of the control loop. The transfer function of the transformer can be determined by recording the input signal and the output signal occurring at the output of the transformer. For this purpose, the signals may be regarded as functions of time as well as, which is more common in this area of technology, as functions of frequency. The latter representation is believed to have advantages in the assessment of the properties of a control loop. The transfer function of a transformer results from the division of the output signal transformed into the frequency range by the input signal transformed into the frequency range. Criteria such as stability and sensitivity with respect to interferences (noise) can be readily assessed using the complex transfer function. Using optimization methods, an optimum parameter set may be found for the control structure by varying the control parameters.
German Published Patent Application No. 103 16 977 describes a method for identifying a control loop in which noise signals covering different frequency ranges are used as input signals. Thus, it is possible to adapt the intensity of every noise signal to the respective frequency range, thereby clearly improving the quality of the identification.
European Published Patent Application No. 1 180 734 describes an identification method using iterative optimization of the control parameters on the basis of quality criteria such as phase margin and amplitude margin. A new identification is performed following each change of the control loop. Since each identification may take a considerable amount of time, such a method is possibly quite time-consuming.